Some information about database access in MediaWiki.By Tim Starling, January 2006.----------------------------------------...
table is like counting beans in a bucket.------------------------------------------------------------------------    Repli...
SELECT queries are the worst offenders should be avoided altogether.Instead do the select first and then the insert.=== Wo...
for it and its schema is considered the canonical version. However,MediaWiki does support the following other DBMSs to var...
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Database

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Database

  1. 1. Some information about database access in MediaWiki.By Tim Starling, January 2006.------------------------------------------------------------------------ Database layout------------------------------------------------------------------------For information about the MediaWiki database layout, such as adescription of the tables and their contents, please see: http://www.mediawiki.org/wiki/Manual:Database_layout http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/maintenance/tables.sql?view=markup------------------------------------------------------------------------ API------------------------------------------------------------------------To make a read query, something like this usually suffices:$dbr = wfGetDB( DB_SLAVE );$res = $dbr->select( /* ...see docs... */ );foreach ( $res as $row ) { ...}For a write query, use something like:$dbw = wfGetDB( DB_MASTER );$dbw->insert( /* ...see docs... */ );We use the convention $dbr for read and $dbw for write to help you keeptrack of whether the database object is a slave (read-only) or a master(read/write). If you write to a slave, the world will explode. Or to beprecise, a subsequent write query which succeeded on the master may failwhen replicated to the slave due to a unique key collision. Replicationon the slave will stop and it may take hours to repair the database andget it back online. Setting read_only in my.cnf on the slave will avoidthis scenario, but given the dire consequences, we prefer to have asmany checks as possible.We provide a query() function for raw SQL, but the wrapper functionslike select() and insert() are usually more convenient. They take careof things like table prefixes and escaping for you. If you really needto make your own SQL, please read the documentation for tableName() andaddQuotes(). You will need both of them.------------------------------------------------------------------------ Basic query optimisation------------------------------------------------------------------------MediaWiki developers who need to write DB queries should have someunderstanding of databases and the performance issues associated withthem. Patches containing unacceptably slow features will not beaccepted. Unindexed queries are generally not welcome in MediaWiki,except in special pages derived from QueryPage. Its a common pitfallfor new developers to submit code containing SQL queries which examinehuge numbers of rows. Remember that COUNT(*) is O(N), counting rows in a
  2. 2. table is like counting beans in a bucket.------------------------------------------------------------------------ Replication------------------------------------------------------------------------The largest installation of MediaWiki, Wikimedia, uses a large set ofslave MySQL servers replicating writes made to a master MySQL server. Itis important to understand the issues associated with this setup if youwant to write code destined for Wikipedia.Its often the case that the best algorithm to use for a given taskdepends on whether or not replication is in use. Due to our unabashedWikipedia-centrism, we often just use the replication-friendly version,but if you like, you can use wfGetLB()->getServerCount() > 1 tocheck to see if replication is in use.=== Lag ===Lag primarily occurs when large write queries are sent to the master.Writes on the master are executed in parallel, but they are executed inserial when they are replicated to the slaves. The master writes thequery to the binlog when the transaction is committed. The slaves pollthe binlog and start executing the query as soon as it appears. They canservice reads while they are performing a write query, but will not readanything more from the binlog and thus will perform no more writes. Thismeans that if the write query runs for a long time, the slaves will lagbehind the master for the time it takes for the write query to complete.Lag can be exacerbated by high read load. MediaWikis load balancer willstop sending reads to a slave when it is lagged by more than 30 seconds.If the load ratios are set incorrectly, or if there is too much loadgenerally, this may lead to a slave permanently hovering around 30seconds lag.If all slaves are lagged by more than 30 seconds, MediaWiki will stopwriting to the database. All edits and other write operations will berefused, with an error returned to the user. This gives the slaves achance to catch up. Before we had this mechanism, the slaves wouldregularly lag by several minutes, making review of recent editsdifficult.In addition to this, MediaWiki attempts to ensure that the user seesevents occurring on the wiki in chronological order. A few seconds of lagcan be tolerated, as long as the user sees a consistent picture fromsubsequent requests. This is done by saving the master binlog positionin the session, and then at the start of each request, waiting for theslave to catch up to that position before doing any reads from it. Ifthis wait times out, reads are allowed anyway, but the request isconsidered to be in "lagged slave mode". Lagged slave mode can bechecked by calling wfGetLB()->getLaggedSlaveMode(). The onlypractical consequence at present is a warning displayed in the pagefooter.=== Lag avoidance ===To avoid excessive lag, queries which write large numbers of rows shouldbe split up, generally to write one row at a time. Multi-row INSERT ...
  3. 3. SELECT queries are the worst offenders should be avoided altogether.Instead do the select first and then the insert.=== Working with lag ===Despite our best efforts, its not practical to guarantee a low-lagenvironment. Lag will usually be less than one second, but mayoccasionally be up to 30 seconds. For scalability, its very importantto keep load on the master low, so simply sending all your queries tothe master is not the answer. So when you have a genuine need forup-to-date data, the following approach is advised:1) Do a quick query to the master for a sequence number or timestamp 2)Run the full query on the slave and check if it matches the data you gotfrom the master 3) If it doesnt, run the full query on the masterTo avoid swamping the master every time the slaves lag, use of thisapproach should be kept to a minimum. In most cases you should just readfrom the slave and let the user deal with the delay.------------------------------------------------------------------------ Lock contention------------------------------------------------------------------------Due to the high write rate on Wikipedia (and some other wikis),MediaWiki developers need to be very careful to structure their writesto avoid long-lasting locks. By default, MediaWiki opens a transactionat the first query, and commits it before the output is sent. Locks willbe held from the time when the query is done until the commit. So youcan reduce lock time by doing as much processing as possible before youdo your write queries.Often this approach is not good enough, and it becomes necessary toenclose small groups of queries in their own transaction. Use thefollowing syntax:$dbw = wfGetDB( DB_MASTER );$dbw->begin();/* Do queries */$dbw->commit();Use of locking reads (e.g. the FOR UPDATE clause) is not advised. Theyare poorly implemented in InnoDB and will cause regular deadlock errors.Its also surprisingly easy to cripple the wiki with lock contention. Ifyou must use them, define a new flag for $wgAntiLockFlags which allowsthem to be turned off, because well almost certainly need to do so onthe Wikimedia cluster.Instead of locking reads, combine your existence checks into your writequeries, by using an appropriate condition in the WHERE clause of anUPDATE, or by using unique indexes in combination with INSERT IGNORE.Then use the affected row count to see if the query succeeded.------------------------------------------------------------------------ Supported DBMSs------------------------------------------------------------------------MediaWiki is written primarily for use with MySQL. Queries are optimized
  4. 4. for it and its schema is considered the canonical version. However,MediaWiki does support the following other DBMSs to varying degrees.* PostgreSQL* SQLite* Oracle* IBM DB2* MSSQLMore information can be found about each of these databases (known issues,level of support, extra configuration) in the "databases" subdirectory inthis folder.------------------------------------------------------------------------ Use of GROUP BY------------------------------------------------------------------------MySQL supports GROUP BY without checking anything in the SELECT clause.Other DBMSs (especially Postgres) are stricter and require that all thenon-aggregate items in the SELECT clause appear in the GROUP BY. Forthis reason, it is highly discouraged to use SELECT * with GROUP BYqueries.

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